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**ECGD 4122 – Foundation Engineering**

Faculty of Applied Engineering and Urban Planning Civil Engineering Department 2nd Semester 2008/2009 ECGD 4122 – Foundation Engineering Lecture 2

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**Revision of Soil Mechanics**

Soil Composition Soil Classification Groundwater Stress (Total vs. Effective) Settlement Strength

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**Soil: A 3-Phase Material**

Air Water Solid

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The Mineral Skeleton Solid Particles Volume Voids (air or water)

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**Three Phase Diagram Air Water Solid Idealization: Mineral Skeleton**

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Fully Saturated Soils Water Solid Mineral Skeleton Fully Saturated

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Dry Soils Air Solid Dry Soil Mineral Skeleton

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**Partly Saturated Soils**

Partially Saturated Soils Solid Air Water Mineral Skeleton Partly Saturated Soils

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**Three Phase System Air Water Solid Volume Weight Va Wa~0 Vv Vw Ww WT**

VT Vs Ws Volume Weight

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**Weight Relationships Weight Components: Weight of Solids = Ws**

Weight of Water = Ww Weight of Air ~ 0

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**Volumetric Relationships**

Volume Components: Volume of Solids = Vs Volume of Water = Vw Volume of Air = Va Volume of Voids = Va + Vw = Vv

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**Volumetric Relationships**

Volume Components: Volume of Solids = Vs Volume of Water = Vw Volume of Air = Va Volume of Voids = Va + Vw = Vv

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**Specific Gravity Unit weight of Water, w**

w = 1.0 g/cm3 (strictly accurate at 4° C) w = 62.4 pcf w = 9.81 kN/m3

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**Specific Gravity, Gs Iron 7.86 Aluminum 2.55-2.80 Lead 11.34**

Mercury 13.55 Granite 2.69 Marble 2.69 Quartz 2.60 Feldspar

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Specific Gravity, Gs

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**Example: Volumetric Ratios**

Determine void ratio, porosity and degree of saturation of a soil core sample Data: Weight of soil sample = 1013g Vol. of soil sample = 585.0cm3 Specific Gravity, Gs = 2.65 Dry weight of soil = 904.0g

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**Example Air Water Solid Wa~0 134.9cm3 W =1.00 243.9cm3 109.0g**

Volumes Weights

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**Example Air Water Solid 134.9cm3 W =1.00 243.9cm3 109.0cm3 585.0cm3**

Volumes s =2.65 341.1cm3 109.0cm3 243.9cm3 134.9cm3 W =1.00

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**Soil Unit weight (lb/ft3 or kN/m3)**

Bulk (or Total) Unit weight = WT / VT Dry unit weight d = Ws / VT Buoyant (submerged) unit weight b = - w

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Typical Unit weights

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**Fine-Grained vs. Coarse-Grained Soils**

U.S. Standard Sieve - No. 200 inches 0.074 mm “No. 200” means...

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**Sieve Analysis (Mechanical Analysis)**

This procedure is suitable for coarse grained soils e.g. No.10 sieve …. has 10 apertures per linear inch

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Hydrometer Analysis Also called Sedimentation Analysis Stoke’s Law

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**Grain Size Distribution Curves**

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Soil Plasticity Further classification within fine-grained soils (i.e. soil that passes #200 sieve) is done based on soil plasticity. Albert Atterberg, Swedish Soil Scientist ( )…..series of tests for evaluating soil plasticity Arthur Casagrande adopted these tests for geotechnical engineering purposes

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**Atterberg Limits Shrinkage limit Plastic limit Liquid limit solid**

Consistency of fine-grained soil varies in proportion to the water content Shrinkage limit Plastic limit Liquid limit solid semi-solid plastic liquid Plasticity Index (cheese) (pea soup) (pea nut butter) (hard candy)

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**Liquid Limit (LL or wL) Empirical Definition**

The moisture content at which a 2 mm-wide groove in a soil pat will close for a distance of 0.5 in when dropped 25 times in a standard brass cup falling 1 cm each time at a rate of 2 drops/sec in a standard liquid limit device

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**Engineering Characterization of Soils**

Soil Properties that Control its Engineering Behavior Particle Size coarse-grained fine-grained Particle/Grain Size Distribution Particle Shape Soil Plasticity

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**Clay Morphology Scanning Electron Microscope (SEM)**

Shows that clay particles consist of stacks of plate-like layers

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**Soil Consistency Limits**

Albert Atterberg ( ) Swedish Soil Scientist ….. Developed series of tests for evaluating consistency limits of soil (1911) Arthur Casagrande ( ) ……Adopted these tests for geotechnical engineering purposes

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Arthur Casagrande ( ) Joined Karl Terzaghi at MIT in as his graduate student Research project funded by Bureau of Public Roads After completion of Ph.D at MIT Casagrande initiated Geotechnical Engineering Program at Harvard Soil Plasticity and Soil Classification (1932)

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Casagrande Apparatus

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Casagrande Apparatus

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Casagrande Apparatus

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**Liquid Limit Determination**

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Plastic Limit (PL, wP) The moisture content at which a thread of soil just begins to crack and crumble when rolled to a diameter of 1/8 inches

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Plastic Limit (PL, wP)

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**Plasticity Index ( PI, IP )**

PI = LL – PL or IP=wL-wP Note: These are water contents, but the percentage sign is not typically shown.

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Plasticity Chart

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**USCS Classification Chart**

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**USCS Classification Chart**

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Plasticity Chart

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**U = porewater pressure = wZw**

Groundwater U = porewater pressure = wZw

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**Stresses in Soil Masses**

P X X Area = A = P/A Soil Unit Assume the soil is fully saturated, all voids are filled with water.

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**′ = - u where, ′ = effective stress**

From the standpoint of the soil skeleton, the water carries some of the load. This has the effect of lowering the stress level for the soil. Therefore, we may define effective stress = total stress minus pore pressure ′ = - u where, ′ = effective stress = total stress u = pore pressure

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**′ = - u Effective Stress**

The effective stress is the force carried by the soil skeleton divided by the total area of the surface. The effective stress controls certain aspects of soil behavior, notably, compression & strength.

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**′z = iHi - u Effective Stress Calculations where,**

H = layer thickness sat = saturated unit weight U = pore pressure = w Zw When you encounter a groundwater table, you must use effective stress principles; i.e., subtract the pore pressure from the total stress.

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Geostatic Stresses

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**Compressibility & Settlement**

Settlement requirements often control the design of foundations This chapter provides a general overview of principles involved in settlement analysis The subject will be dealt with in greater detail in Chapter 7.

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**Increase in Vertical Effective Stress**

Due to a Placement of a fill Due to an external load

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**}z f′ }z f′ Consolidation z0′ z′ z0′ z0′ z0′ z′ Before After**

H z0′ }z f′ z0′ z′ Voids e Vv = eVs Vv = (e - e)Vs Voids Solids Solids Before Vs Vs After

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Before Loading Point, P u0 0

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**Immediately After Loading**

Point, P u0+u 0 +

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**Shortly after Loading No settlement Long after Loading**

0 + Long after Loading Settlement Complete u0+u u0 0 +

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**Settlement Distortion Settlement (Immediate)**

Consolidation (Time Dependent) Secondary Compression Time Settlement

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**Laboratory Consolidation Test**

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Consolidation Test

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Test Results

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Consolidation Plot

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Idealized Data Test Results

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**Compression Index and Recompression Index**

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**Compression Ratio and Recompression Ratio**

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**Normally and Over-Consolidated Soils**

….. Normally consolidated ….. Over consolidated ….. Under consolidated

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**Over-Consolidation Margin & Over-consolidation Ratio**

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**Typical Range of OC Margins**

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**Compressibility of Sand and Gravels (Table 3.7)**

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Example 3

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**Settlement Predictions N.C. Clays**

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**Settlement Predictions O.C. Clays…… Case I**

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**Settlement Predictions O.C. Clays…… Case II**

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Example 4

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Example 4

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Example 5

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Example 5

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**Failure due to inadequate strength at shear interface**

Slope Failure in Soils Failure due to inadequate strength at shear interface

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Shear Failure in Soils

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Shear Failure in Soils

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**Bearing Capacity Failure**

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**Transcosna Grain Elevator Canada (Oct. 18, 1913)**

West side of foundation sank 24-ft

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**Shear Strength of Soils**

Soil derives its shear strength from two sources: Cohesion between particles (stress independent component) Cementation between sand grains Electrostatic attraction between clay particles Frictional resistance between particles (stress dependent component)

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**Shear Strength of Soils; Cohesion**

Dry sand with no cementation Dry sand with some cementation Soft clay Stiff clay

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**Shear Strength of Soils; Internal Friction**

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**Mohr-Coulomb Failure Criterion**

Shear Strength,S = C Normal Stress, =

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**Shear Strength is controlled by Effective Stress, '**

Slope Surface Potential Failure Surface

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**Mohr-Coulomb Failure Criterion**

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Typical Values

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**Effect of Pore Water on Shear Strength**

Pore water pressure Total Stress, versus Effective Stress, Shear Strength in terms of effective stress

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**Apparent Cohesion Moist beach sand has apparent cohesion**

Negative pore water pressures

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**Measuring Shear Strength**

Laboratory Direct shear test Unconfined compression test Triaxial compression test Field Vane shear test

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**Direct Shear Test ASTM D-3080; AASHTO T 236**

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Direct Shear Test

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Direct Shear Test

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**Direct Shear Test Device**

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**Direct Shear Test Device**

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Direct Shear Test Data Shear stress

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**Direct Shear Test Data Volume change**

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**Peak vs. Ultimate Strength**

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**Example: Direct Shear Test**

Given: A direct shear test conducted on a soil sample yielded the following results: Normal Stress, (psi) Max. Shear Stress, S (psi) 10.0 6.5 25.0 11.0 40.0 17.5 Required: Determine shear strength parameters of the soil

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Example 6

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**Drained versus Undrained Conditions ….**

Before loading After loading

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**Drained versus Undrained Conditions ….**

Before loading After loading

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**Soil Shear Strength under Drained and Undrained Conditions ….**

Drained conditions occur when rate at which loads are applied are slow compared to rates at which soil material can drain Sands drain fast; therefore under most loading conditions drained conditions exist in sands Exceptions: pile driving, earthquake loading in fine sands

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**Soil Shear Strength under Drained and Undrained Conditions ….**

In clays, drainage does not occur quickly; therefore excess pore water pressure does not dissipate quickly Therefore, in clays the short-term shear strength may correspond to undrained conditions Even in clays, long-term shear strength is estimated assuming drained conditions

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**Shear Strength in terms of Total Stress**

Shear Strength in terms of effective stress Shear strength in terms of total stress u at hydrostatic value

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Long-term Stability Potential Failure Surface Slope Surface

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Short-term Stability Potential Failure Surface Slope Surface

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**Shear Strength in terms of Total Stress; = 0 condition**

For cohesive soils under saturated conditions, = 0.

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**Mohr-Coulomb Failure Criterion**

Shear Strength,S = 0 C Normal Stress,

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Mohr’s Circles 3=0 1 Direct Shear Uniaxial Compression

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**Mohr’s Circles 1 3=0 1 Uniaxial Compression Max. shear plane**

Horiz. plane 1

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Mohr’s Circles 3=0 1 Uniaxial Compression

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**Unconfined Compression Test ASTM D-2166; AASHTO 208**

3 = 0 1 For clay soils Cylindrical specimen No confining stress (i.e. 3 = 0) Axial stress = 1

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**Unconfined Compression Test Data**

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**Unconfined Compression Test**

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**Example: Unconfined Compression Test**

Given: An unconfined compression test conducted on a soil sample yielded the results shown in the table. Required: Determine undrained shear strength, Su of the soil

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**Example: Unconfined Compression Test**

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**Example: Unconfined Compression Test**

Su= 21.7psi = 3128 psf qu= 43.45psi=6257 psf

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**Triaxial Compression Test**

Unconfined compression test is used when = 0 assumption is valid Triaxial compression is a more generalized version Sample is first compressed isotropically and then sheared by axial loading 1 3

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**Triaxial Compression Test**

1 3 Load applied in 2 stages confining pressure, 3 dev. stress, = 1 - 3

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**Triaxial Compression Test**

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**Triaxial Compression Test**

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**Triaxial Compression Test for Undisturbed Soils**

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**Drainage during Triaxial Compression Test**

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**Triaxial Compression Tests**

Unconsolidated Undrained (UU-Test); Also called “Undrained” Test Consolidated Undrained Test (CU- Test) Consolidated Drained (CD-Test); Also called “Drained Test”

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**Triaxial Compression Tests ASTM Standards**

ASTM D2850: Unconsolidated Undrained Triaxial Test for Cohesive Soils ASTM D4767: Consolidated Undrained Triaxial Test for Cohesive Soils

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**Triaxial Compression Tests AASHTO Standards**

AASHTO T-296: Unconsolidated Undrained Triaxial Test for Cohesive Soils AASHTO T-297 : Consolidated Undrained Triaxial Test for Cohesive Soils

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**Consolidated Undrained Triaxial Test for Undisturbed Soils**

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